To meet current emission regulations, automotive vehicles must regulate the air/fuel ratio (A/F) supplied to the vehicles' cylinders so as to achieve maximum efficiency of the vehicles' catalysts. For this purpose, it is known to control the air/fuel ratio of internal combustion engines using an exhaust gas oxygen (EGO) sensor positioned in the exhaust stream from the engine. The EGO sensor provides feedback data to an electronic controller that calculates preferred A/F values over time to achieve optimum efficiency of a catalyst in the exhaust system. It is also known to have systems with two EGO sensors in the exhaust stream in an effort to achieve more precise A/F control with respect to the catalyst window. Normally, a pre-catalyst EGO sensor is positioned upstream of the catalyst and a post-catalyst EGO sensor is positioned downstream of the catalyst. Finally, in connection with engines having two groups of cylinders, it is known to have a two-bank exhaust system coupled thereto where each exhaust bank has a catalyst as well as pre-catalyst and post-catalyst EGO sensors. Each of the exhaust banks corresponds to a group of cylinders in the engine. The feedback signals received from the EGO sensors are used to calculate the desired A/F values in their respective group of cylinders at any given time. The controller uses these desired A/F values to control the amount of liquid fuel that is injected into the cylinders by the vehicle's fuel injector. It is a known methodology to use the EGO sensor feedback signals to calculate desired A/F values that collectively, when viewed against time, form A/F waveforms having ramp portions, jumpback portions and hold portions, as shown in FIG. 3.
Sometimes, in a two-bank, four-EGO sensor exhaust system, one of the pre-catalyst EGO sensors degrades. In such case, it is desirable to be able to control the A/F in the group of cylinders coupled to the exhaust bank having only one operational EGO sensor by using the feedback signals received from the three operational EGO sensors alone. It is a known methodology to compensate for a degraded pre-catalyst EGO sensor in one of the exhaust banks by having the A/F values in the corresponding group of cylinders mirror the A/F values in the other group of cylinders. Essentially, this known methodology simply calculates desired A/F values over time for the group of cylinders coupled to two properly functioning EGO sensors and uses those A/F values for both banks. But this methodology fails to utilize the feedback signal provided by the post-catalyst EGO sensor in the exhaust bank having the degraded pre-catalyst EGO sensor. Therefore, the A/F values applied to the group of cylinders coupled to the degraded pre-catalyst EGO sensor do not benefit from any feedback signal specific to that bank, and, as a result, the A/F values used in that group of cylinders may not be optimal to enable the corresponding catalyst to perform most efficiently.
Therefore, it is desirable to have an improved methodology and system for calculating A/F values for a group of cylinders coupled to an exhaust bank having a degraded pre-catalyst EGO sensor. The improved methodology and system should utilize the feedback signal received from the post-catalyst EGO sensor in the exhaust bank having the degraded pre-catalyst EGO sensor to calculate more responsive A/F values and thus enable the catalyst to operate more efficiently.